1. Field of the Invention
The present invention relates to a variable focal length lens barrel.
2. Related Background Art
Japanese Patent Application Laid-Open Publication No.10-31150 discloses a lens group moving mechanism for a variable focal length lens in the prior art, which is constructed to attain a decrease in thickness of a camera by increasing a moving quantity for a collapsing operation without any rises both in length and diameter of a lens barrel. A configuration of this mechanism is that a width-enlarged portion with an enlarged width of a thread root is formed in a part of a female helicoid thread of a multi-streaks thread, and a dimension of length of the width-enlarged portion in a direction along the helix Is equalized to a length of crest of a male helicoid thread in the direction along. the helix, which meshes with the female helicoid thread. With this configuration, it is possible to obtain thread-fitting having a comparatively small lead at a portion with a non-enlarged width of the root of the thread, and a thread fitting having a comparatively large lead at the portion with the enlarged-width of the root. That is, a dimension in section right-angled to the helix with respect to the crest of the male helicoid thread is defined as a "width", while a length in the direction along the helix is defined as a "length". In this case, the male helicoid thread and the female helicoid thread mesh with each other over the "width" or the "length" thereof. Then, in the case of meshing over the "width", there is acquired a thread fitting state having a comparatively small lead. In the case of meshing over the "length", there is obtained a thread fitting state having a comparatively large lead. The complex lead threads having two leads different from each other can be obtained in the way described above.
Further, Japanese Patent Application Laid-Open Publication No. 3-209445 discloses a technology of reducing the number of parts of the helicoid thread and the cam for moving the lens groups. According to this technology, one single cam ring is formed with both a cam groove and a lead thread.
If the complex lead thread and the cam groove are formed in overlap in an internal peripheral surface of a cylindrical member of the single cam ring, the cam groove might extend across a portion where the lead changes, i.e., a bending portion of a root of the female helicoid thread. At this time, a side wall of the bending portion of the helicoid thread is separated by the cam groove, resulting in a deficient state.
When the crest of the male helicoid thread meshing over the "width" is on the verge of reaching the separated bending portion, with the bending portion being in the as-deficient state, the same crest advances rectilinearly without being changed to the state of meshing over the "length", consequently comes off the root of the female helicoid thread, and drops down to the cam groove. As a result, a rotation of the cam ring is locked, and the variable focal length lens might be incapable of performing a zooming operation in some cases.